In cryogenic refrigerators such as Stirling and Gifford-MacMahon type refrigerators, a piston-like displacer reciprocates within a cylinder. For efficient refrigeration in those two systems, a gas seal is provided between the displacer and cylinder to assure that refrigerant passing from one end of the displacer to the other passes through a regenerator in the displacer. One type of seal which has been widely used in such refrigerators is the split ring seal having a Z-cut there across to permit circumferential expansion of the ring without loss of sealing. The seal is positioned in a circumferential groove in the displacer and is pressed outward against the cylinder by an inner expander ring. When assembled, the split seal fits snugly within the groove. A primary advantage of such a seal in cryogenic refrigerators is that, even with wear and thermal contraction or expansion of the seal, the expander assures a fairly constant sealing force on the cylinder wall.
As shown in FIG. 1, a typical split Gifford-MacMahon or Stirling refrigerator includes a displacer 12 which reciprocates in a cylinder 14. The displacer is driven by a motor or gas spring volume through a piston rod 13 and pin connection 15. Upward movement of the displacer 12 causes high pressure gas in a warm chamber 16 to be displaced through a regenerator 18 within the displacer. The gas passes outward through a porous metal plug 20 or side ports (not shown) into a cold end expansion chamber 24. The thus cooled gas is expanded in the chamber 24 to further cool the gas and surrounding high conductivity heat station 26. Then, with downward movement of the displacer the very cold gas is returned through the regenerator 18 to cool that regenerator for cooling of gas in the next cycle of operation. To assure that all refrigerant, such as helium gas, is directed through the regenerator with movement of the displacer, the seal ring 30 is positioned in a peripheral groove in the displacer near its warm end.
The seal ring 30 is generally a split ring of plastic material such as fiber glass tetrafluoroethylene (TFE), whereas an expander ring 38 is of spring steel and the groove is usually formed in stainless steel. Thus, the seal ring 30 has a much greater coefficient of thermal expansion than do the surrounding metal parts. To allow for thermal expansion of the seal ring within the groove, rings in conventional refrigerators have a slightly smaller axial dimension than that of the groove to within 0.5 mils. Thus, except when operating at high temperatures, the seal ring shuttles within the groove along the expander ring with each change in direction of the displacer movement. During the time intervals that the seal is shifting within the groove, there is no compressive force to maintain a tight seal along a circumferential cut or Z-cut in the seal ring 30 and at a groove face. Thus, leakage is experienced with each change in direction of the displacer. Also, the seal ring must alternately seal against opposite faces of the groove. For this sealing, the faces of the seal ring and the groove must be precisely smooth, flat and parallel. Such necessary precision, makes large scale fabrication of consistently and uniformly operating devices difficult.
When the refrigerator is exposed to a cold environment, or the seal is cooled by the refrigerator itself, the seal ring shrinks and, as a result, shuttles to a greater extent along the expander ring with each reciprocating movement of the displacer. Such shuttling of the seal within the groove results in greater wear and leakage. The wear of the seal ring leads to even greater leakage around the displacer and also gives rise to debris which, when mixed with the helium refrigerant, reduces the efficiency of the refrigerator.
In U.S. Pat. No. 4,355,519, herein incorporated by reference, shuttling of the seal was prevented by the use of a spring within the displacer to produce an axial load on the seal ring. The use of a Belleville washer and an annular spring having a u-shaped or c-shaped cross section were described.
In U.S. Pat. No. 6,408,631, herein incorporated by reference, a wave spring produces the axial load on a seal ring in a piston.
U.S. Pat. No. 6,598,406, herein incorporated by reference, illustrates the use of load rings in combination with an elastomer o-ring to minimize leakage at the seal ring. Figure FIG. 2A is taken from U.S. Pat. No. 6,598,406. FIG. 2B is a cross-sectional view of the same seal assembly. A wave spring 48 provides an axial load on a load ring 144, an elastomer O-ring 143, and another load ring 44, which in turn provides an axial load onto a dynamic seal 46.